In the construction of tunnels it is a common practice to provide a tunneling machine which excavates the material ahead of the machine and as its excavating head is driven forwardly by hydraulic or other fluid-operated units braced with respect to the tunnel wall via shields or the like which can form a shell engaging the tunnel wall behind the head.
It is frequently desirable to line the tunnel wall behind the machine with concrete, i.e. to emplace concrete in a pumped form as a lining material along this wall.
While concrete may be sprayed on the wall for this purpose, in many cases the most convenient, economical and desirable technique has been to construct an inner form or casing at a given spacing from the wall of the tunnel and to pump concrete into the resulting space.
At the forward end of this space, a formface can be moved in the direction in which the lining is propagated. This face can be sealed with respect to the inner form or casing and can move with an outer casing which can form part of or be affixed to the shield mentioned previously.
As noted, the shield shell or casing can be connected to the head by piston-and-cylinder arrangements which are articulated to the shield and to the excavating head and which serves to advance the head and to draw the shield toward the head, e.g. in successive or alternating operations. Where the face of the concrete mold constitutes part of this shield, it is drawn forwardly with advance of the concrete and the concrete is pumped behind this shield.
Since the shield is drawn forwardly and the concrete is pumped into the space which results as the shield is drawn forwardly, a constant concrete pressure cannot be maintained at the face of the advancing form. This can result in defects in the concrete lining of the tunnel.
For example, when the strata through which the tunnel is propagated has water pockets or a structure in which water is trapped or which may be permeated by water, any free space which may result from the advance of the shield of the formface as it is drawn forwardly, may result in the drawing of water into the space and the dilution of the concrete or the formation of pockets therein.
In some cases, free water may develop in the space which cannot be expressed by the concrete pumped into the space so that water pockets may remain as defects in the concrete. Consequently, the independent operation of the advance of the formface and the concrete pump almost invariably will lead to defects and in the case in which the tunnel structure has a high water content, may give rise to very significant problems.
To reduce these problems, it has been proposed to measure the pressure of the concrete pumped into the space directly behind the formface and/or in the concrete supply pipe ahead of the formface and to utilize the measured value as a control for the operation of the hydraulic system for advancing the formface and for operating the concrete pump.
When the pressure of the concrete exceeds a limiting value, the concrete supply is throttled. When a lower limit is reached, the hydraulic medium feed to the hydraulic advancing units for the formface is reduced and the advance is slowed.
While this system can to some extent reduce the detrimental effect upon the lining and thus eliminate some of the disadvantages of the earlier systems as described, it has its own disadvantage which is that it is relatively slow to react since control corrections will depend only upon crossing of threshold values. Since the threshold values are spaced apart to define a range of operative concrete pressures, volume differences cannot be avoided within this range.
It has also been found that even with the controls described previously, significant pressure drops in the liquid concrete in the space may result with incursion of loose material from the tunnel wall into the lining.